Laser marking apparatus and methods

ABSTRACT

A method and apparatus for laser marking a moving workpiece such as a wire or cable directs a substantially constant rapidly pulsed laser beam at a frequency of at least 15 Hz towards a low inertia, low mass rotatable mask with character apertures disposed around its periphery. The mask is driven asynchronously to present successive characters with each pulse to form a composite image.

This invention relates to laser marking apparatus and methods and inparticular, but not exclusively, the invention is concerned with markinga series of marks or characters on an elongate and/or moving object,such as a wire, cable or tube, or on a series of objects.

Manufacturers of products containing a large number of electrical wiresor cables, or other elongate elements such as pipes or tubes, are oftenobliged to mark them with identification codes for production,maintenance and safety reasons. This is particularly so in the aerospaceindustry where the outer insulation of wires must be marked at regularintervals along their length (typically every 75 mm). Historically, thishas been achieved using hot stamp, ink jet and, more recently, lasertechnologies. The word "wire" is used to include the electricalconductor, and any insulation or shielding.

For the last several years, ultra violet (U.V.) laser wire markers havebeen available based on a specific type of pulsed U.V. laser known as an"Excimer" laser which generally induces a colour change in TitaniumDioxide, which is contained as a pigment in many plastics materials usedas insulation. These U.V. laser wire markers are quickly becoming thepreferred solution in higher volume manufacturing situations because oftheir speed of operation and particularly because they produce highquality permanent marks on the most advanced, "non-stick", thin-walledfluoropolymer finished wires and cables without damaging or affectingthe integrity of the insulation.

In a typical arrangement, radiation from an excimer laser which can befired on demand is used to illuminate alphanumeric characters on acontinuously rotating mask. The laser, being capable of asynchronousoperation, is slaved to the mask and fired when the selected characteror the mask is in the path of the laser beam. De-magnified images of themask characters are created on the surface of the wires therebyproducing wire or cable identification marks. To allow for the fact thatthe laser does not fire at regular intervals when a series of charactersis printed, a galvanometer mirror is positioned to provide a variabledeflection to ensure constant character spacing and to increaseeffective throughput. This system gives excellent results and hasachieved substantial commercial success with major aerospacemanufacturers, but the system is sophisticated and the purchase andrunning costs reflect this. Excimer lasers are large, expensive, usetoxic gases to produce the laser radiation and require special services.Thus the site for the laser marking machine requires water or aircooling services, an extraction system, a source of compressed air, asupply of several gases, and a suitable power supply. However, againstthis these systems do have the advantage that the laser is slaved to themask and so the mask is only required to rotate at a constant speed.

Naturally, in any machine, particularly those without galvanometers, thewire marking speed is extremely important because this dictates theproductivity of the machine, with a typical throughput for markingapplications needing a laser firing at 20 Hz. Any slower than this wouldmean that the throughput rate of wire was unacceptably low. Existingmachines using fire-on-demand lasers employ a variety of techniques tomaximise this speed, including sweeping the beam to track the wireduring printing. To cater for a variety of sizes of wire, and customerrequirements, the rotating mask usually needs to carry several printsizes and also to present characters in both vertical and horizontalorientations, and so the mask may carry three or more character setsaround its periphery, which in turn dictates the diameter and inertia ofthe mask.

A need exists for a laser marking machine of simpler construction forlower volume manufacturing and maintenance operations and which does notplace such high demands in terms of the services required at the site.

In our first proposal, we investigated the possibility of a markingsystem which used a pulsed solid state laser instead of an excimerlaser. Pulsed solid state lasers have the advantages of lower cost thanexcimer lasers and minimal service requirements but their mode ofoperation is synchronous, meaning that the rotatable mask must be slavedto the laser, requiring the mask to be rotated in different directionsand at different speeds. However, this appeared to present insuperableproblems in terms of moving the mask quickly and accurately enough to beready for the next laser pulse. Given a typical existing rotary mask andthe capabilities of typically available stepper motors it appeared thatthe best that could be achieved was a cycle of 80 ms for a half rotationof the mask which, allowing for settling, would mean that a markingspeed of little more than 10 Hz would be possible, which would beunacceptable for many purposes.

However, we have found that, by redesigning the mask, special control ofthe motor, and optional optical transformation means for opticallytransforming the mask characters to reduce the number of characters onthe mask and thus its diameter, mass and inertia, it is possible toprovide a laser marking system which provides marking rates of 20 Hz ormore.

Accordingly, in one aspect of this invention, there is provided a lasermarking apparatus comprising:

a source of pulsed laser radiation for producing a beam of radiation,

a rotatable mask for being illuminated by said radiation beam andincluding a plurality of character apertures spaced angularly around theaxis of rotation thereof, and

drive means for rotating said rotatable mask,

wherein said drive means is operable intermittently to index saidrotatable mask successively to align selected character apertures withsaid laser beam.

In embodiments of apparatus of this aspect of the invention, a rotatablemask is driven discretely to interpose the required character aperturesin the radiation beam. Although the laser source could comprise anysuitable laser source, the invention has been made with particularreference to a pulsed, solid state type of laser, e.g. an Nd:YAG laser,which is pumped by a flash lamp or the like to provide pulses of laserradiation at a substantially constant pulse rate. The pulse rate ispreferably at least 15 Hz and more preferably 20 Hz or more. The use ofthe term `pulsed` refers to repeated energisation of the lasing mediumto provide the appropriate thermal environment; it does not require thatthe laser source necessarily emit radiation at each pulse. For exampleif a "blank" output pulse was required the laser shutter may be keptclosed for the duration of the pulse.

In a preferred embodiment, the laser source includes one or morefrequency multipliers to provide an output beam in the U.V. waveband ofwavelength from 200 nm to 400 nm to create a marking fluence of betweenabout 50 mJcm⁻² and about 2000 mJcm⁻². The frequency multiplying meanspreferably multiplies the frequency by a multiple of three, althoughother multiples are possible.

The drive means preferably comprises a stepper motor and control meansfor providing a selected drive input profile to said stepper motor tomove the mask between successive angular positions. The input profilepreferably includes an acceleration phase, a substantially steady speedphase, a deceleration phase and a settling phase, selected with regardto the physical characteristics of the mask, the responsecharacteristics of the stepper motor, and the pulse rate of the laser,to ensure that the mask is ready with the correct character aperture inthe beam path when the next pulse is generated.

The drive means preferably includes means for storing, for eachincrement of angular movement of said mask, a respective drive profilefor being applied to said stepper motor. In this way, the speed of whatmay be termed the pulse profile is matched to the mask and motorcharacteristics. The acceleration and deceleration speeds and durationsfor each possible "jump" between characters may be stored for each jumpfrom a single character to a 180° jump (e.g. 25 characters). The steppermotor may be driven in either direction and the profile for a clockwisejump may be the same as that for an anticlockwise jump.

The stepper motor is preferably a rare earth magnetic stepper motor withthe coils driven in parallel.

Although the mask may carry various character sets, it is preferred toreduce or minimise the inertia of the mask by including only onecharacter set on the mask. Whilst the mask inertia should be kept as lowas possible, the mask should have sufficient material to ensure that itis not subject to excessive transient movement when rapidly deceleratedor ablated around the character apertures by the laser beam. A typicalexample of a mask is made of thin (about 0.08 mm) stainless steelmaterial, and of diameter around 67 mm, although different dimensionswill of course apply for different numbers of characters in the set,different materials etc.

Normally, the provision of just one character set on the mask would notbe commercially acceptable because users wish to have fonts of differentsizes, and of vertical and horizontal orientations, to provide suitablemarking on wires or tubes of different diameters.

The apparatus therefore preferably includes optical transformation meansin the beam path beyond the mask, operable to select the size and/ororientation of a character at the marking plane. The opticaltransformation means may include one or more lens means of selectedoptical power which may be moved into the beam to adjust the size of theimage. It may also include one or more mapping means, for exampleprisms, which may be moved into the beam to present the character in avertical, horizontal or other orientations. The mapping means preferablycomprises two dove prisms to present the characters in the vertical andthe horizontal orientation respectively.

By the use of the optical transformation means, the system may providean extended character range whilst employing a mask of very low mass andinertia. For example, the mass of a mask used in embodiments of thisinvention, excluding the rotary hub, may be well under 5 gms andtypically 2 gms or less and ideally less than 1.5 gm. This compares witha mask mass of 35.2 gm for an existing excimer laser wire marker. In oneembodiment, the optical transformation means can apply four differentcombinations of size and orientation, thus providing four usablecharacter sets (each of 50 characters) from the one on the mask, and atotal character count of 200. The mass per usable character maytherefore be as low as 6.5 mg, compared to about 294 mg for aconventional mask. The mass reduction ratio for the mask foil istherefore about 45:1 and the inertia reduction about 237:1.

The apparatus preferably includes means for advancing the wire or cableto be marked through the apparatus. This function may he achieved in avariety of ways in the apparatus itself or upstream/downstream thereof.For greater throughput, the means for advancing is preferablycontrollable to allow the wire or tube speed between identificationmarks to be increased.

In another aspect, this invention provides a laser marking method whichcomprises marking a series of characters on a workpiece by means of abeam of laser radiation which is pulsed at a generally constant pulserate and a rotatable mask carrying a set of mask character aperturesthereon, wherein the mask is indexed discretely to align successivecharacter apertures with the laser beam.

In a further aspect of this invention, we have developed a laser markingor machining system which does not rely either on a mask imagingtechnique or a scanning spot technique. The mask imaging technique hasbeen discussed above. In the latter technique, a laser beam iscontrolled by one or more galvanometer mirrors to write or draw on atarget surface, e.g. X-Y fashion or as a raster scan.

Neither technique can be claimed to be predominantly employed over theother as each has its own benefits, restrictions or limitations. Maskimaging has the benefits of requiring only one laser shot per characterand generates a real image of the mask, with detail limited by theprojection optics and mask fabrication. It requires only mediumrepetition rate lasers to provide accepted production rates. However,requiring a solid mask means mask changeover or mark flexibility isrestrictive and/or slow. Because of this, commercial marking systemshave fixed, limited character set capabilities.

Notwithstanding the quality of the mark, the performance of lasermarking machines is measured on wire throughput--the amount of wirewhich can be processed in a given time. Throughput can be considered tobe inversely proportional to two simple variables--the laser charge time(Tc) between shots and the mask changeover time, or mask latency (Tm).On state-of-the-art excimer laser wire markers Tc is of the order of 5ms and Tm is of the order of 3 ms producing an undesirable delay of 8 msbetween shots or marks on the wire. Certain steps may be employed toslightly reduce this delay but the total delay is not reducedsignificantly.

The 3 ms mask access time may be reduced by decreasing the mask size orby reducing the number of available characters or character sets.Unfortunately, current trends require an expanded character set meaninga larger mask and therefore increasing the mask access time. It is notpossible to simply increase rotational mask speed as the delicate maskcannot sustain the high centrifugal forces.

The 5 ms laser charge time is state-of-the-art for U.V. wire markinglasers and a large decrease in Tc is not anticipated in the near future.

On the other hand, scanning mirror-type markers require high repetitionrate lasers to achieve industrial marking requirements. The laser spotis a fixed size which can limit resolution and hence character detail,but most restrictive is the speed and precision of control. In mostinstances scanning systems will be chosen over mask-based systems whererequirements demand flexibility of control to generate a limitlesscharacter set, and the ability to mark or machine over large targetareas.

We have developed a laser marking or machining apparatus designed toobviate at least some of the above problems.

Accordingly, in another aspect of this invention, this inventionprovides a laser marking or machining apparatus, comprising a source oflaser radiation for producing a beam of radiation, and spatial lightmodulator means for modulating said beam to produce an image at themarking or machining plane.

In this apparatus, the image may be created by a solid state modulatorthus avoiding the constraints posed by a rotating mask or a scanningmirror.

The spatial light modulator may take many forms but a preferred form ofmodulator is the digital micromirror device (DMD), examples of which areproduced by Texas Instruments Inc. These devices are described in"Micromirrors and Digital Processing: Bringing a New Look to Displays":G. A. Feather, Photonics Spectra, May 1995, pp118-124, the contents ofwhich are incorporated herein by reference.

The spatial light modulator preferably comprises an array ofindividually addressable pixels together making up the image formed orexposed at the marking or machining plane. In a typical example of suchan array there may be 864×576 pixels making up nearly 500,000 in an areaof 100 mm². Since the mask "changeover mechanism" employs switching ofpixels rather than mechanical movement, extremely fast mask access timesare possible with spatial light modulator micromirrors. For example,digital micromirror devices switch in approximately 10 μs, thusimproving changeover by a factor of 300. Since each micromirror isindividually controllable, this presents a considerably increasedcharacter set, which is a vast improvement from the typical set of 50characters of current state-of-the-art markers.

The impact of speed capability from using micromirrors is easilyappreciated but the flexibility of the character set is also asubstantial benefit. Wires used in the aerospace industry are of acontinuous diameter range, with specifications for marking usuallyinsisting on an approximate 80% fill factor (character size to wirediameter). On the smallest wires print font may be switched fromhorizontal to vertical to maintain a standard 4:3 height:width aspectratio thus ensuring optimum legibility even on small gauge wires. Withfixed metal mask systems it is prohibitively costly to provide acontinuum of available print sizes to match this wide range of wirediameters. However, with the considerable flexibility of a programmablearray of light modulation pixels, for example 500,000 micromirrors, the80% fill factor should be achievable in all current and futureanticipated wire sizes with horizontal or vertical or italic fontsequally achievable and accessible.

In this latter respect, aerospace manufacturers have noted theimportance of distinguishing those marks applied during manufacture ofthe wire from those functional identifying marks applied later by thecustomer. This is currently done by the inkjet/laser mark differentialbut will almost certainly require another distinguishing feature aslaser marks replace inkjet marks for wire manufacture codes.

Spatial light modulators or micromirrors will also enable slightmodifications of letter positions such as superscripts or subscripts butthis lateral displacement capability will also be of great use forproviding an electronic rather than mechanical method of preciselycentering the identifying marks in the centre of the wire.

In another aspect, this invention provides adjustment of the imagewritten by said modulator to compensate for movement of the item to bemarked or machined relative to the marking or machining plane.

Finally, the use of spatial light modulators provides one very importantfurther capability, namely printing machine readable codes.

In quality driven environments such as aviation wire marking there is adefinite move from human readable alphanumerics to machine readablecodes. Existing industrial standards are being adopted, but are notsuitable for wires, or in general small parts. For example, Bar Code 39is a linear bar code which when currently generated by a laser markerrequires a particularly slow speed because of the spatial precision anddensity of information requiring several, e.g. 5, shots per character.The result is a long code which, on a wire or on a small part requires alarge space which may not be available. This presents problems whenattempting to read the code back because of twisting and alignmentaccuracy. We believe that there is a requirement for a high resolutioncode with a higher density, thereby occupying a smaller area.

Spatial light modulators, and in particular DMDs can be programmed toform a two dimensional dot matrix code which would mean, in a singlelaser shot, a substantial amount of machine readable code could bemarked in an extremely small area. Linear spatial accuracy could beeliminated or reduced as a problem. Wire throughputs would undergo aphenomenal increase over alphanumeric code printing which are themselvesorders faster than bar code printing. And with half a million individualmirror pixels addressable provides enough information content for mostcoding requirements. With reading back codes, twisting of wires would beof reduced importance as an information block is a single matrix entityrather that an elongated message.

Accordingly, in a further aspect of this invention there is provided amethod of marking a component, which comprises passing a beam of laserradiation to a spatial light modulator and thence to said component, andcomposing on said spatial light modulator an intermediate imagecomprising a block or patch comprising a plurality of characters ormarks. Thus, the modulator beam will mark the wire with said block orpatch.

Whilst the invention has been described above, it extends to anyinventive features set out above or in the following description.

Two embodiments of the invention will now be described by way of exampleonly, reference being made to the accompanying drawings, in which:

FIG. 1 is a block diagram of a wire marking system in accordance withthis invention;

FIG. 2 is a schematic diagram showing the optical configuration of afirst embodiment of laser marking system;

FIG. 3 illustrates the four character sets available from the embodimentof FIG. 2,

FIG. 4 is a diagram showing a typical accelerate/sustain/deceleratephase for one of the increments of jumps of the mask stepper motor usedin the apparatus of FIGS. 1 and 2, and

FIG. 5 is a schematic diagram showing the optical configuration of asecond embodiment of laser marking system.

Referring initially to FIG. 1, both embodiments described below employsimilar wire handling schemes. A drum 10 containing the wire 12 to bemarked is positioned on the dereeler 14 and the wire fed through thewire drive assembly 16 to the coiling pan 18. The wire drive assembly 16includes a tractor drive mechanism 20 powered by a stepper motor 22through a gearbox (not shown). The wire throughput rate varies dependingupon the required intercharacter spacing and the space betweenidentification marks. The system is controlled by an IBM Compatible PC24 using simple menu driven software. A solid state Nd:YAG laser 26 ispumped to emit pulses at a generally fixed repetition rate f (typicallyf=20 Hz for the illustrated embodiments). Each character must be placedin the path of the laser beam in a time significantly less that 1/f. Inorder to achieve this two different embodiments are proposed.

The configuration of the marking system of the first embodiment is shownin FIG. 2. The output beam from the laser 26 is directed via mirrors M1and M2 to a mask assembly indicated generally at 28. Here the beam istruncated by a mask aperture (not shown) to remove unwanted radiationfrom the beam and to allow the remainder to efficiently illuminate themask characters (or apertures). The beam then passes through one of twoprisms (P1 or P2), which rotates the image of the mask character through90° or 180° as required. A lens (L1 or L2) is used to form an image ofthe mask character on the wire surface. Combinations of these lenses andprisms enable two different font sizes and vertical and horizontalorientations to be marked, as illustrated in FIG. 3.

The mask assembly 28 comprises a rotatable stainless steel disk mask 30which contains just one set of 50 alphanumeric characters (A . . . Z, 0. . . 9, and a selection of symbols). A stepper motor 32 is used to movethe mask 30 between laser pulses to position each character aperture inplace before the next laser pulse arrives. In order to achieve this, ahigh speed stepper motor, and a special drive system is used, and theinertia of the mask minimised. The stepper motor 32 is a high power unitusing rare earth magnets and having its coils driven in parallel, with100 steps producing the full 360° rotation. An example of a suitablemotor is an ESCAP (RTM) motor reference P532-258 004 available fromMcLennan Servo Supplies, Yorktown Industrial Estate, Camberley, Surrey,UK.

The computer 24 and its interface card(s) generate a series of pulseswhich drive the mask stepper motor 28. The frequency of these pulses isdynamically varied throughout a `jump` from one mask character toanother to achieve a fixed movement within the required timeframe. Theacceleration and deceleration for each `jump` on the mask from onecharacter to another is optimised to the length of the `jump`, theweight/inertia of the mask and the characteristics of the motor 28.Table 1 below indicates the pulse profiles for three character jumps,corresponding to a minimal jump (just one character), 72° (tencharacters) and 180° (twenty-five characters), and FIG. 4 illustratesthe acceleration/sustain/deceleration profile.

    ______________________________________                                                            Duration of Phase                                         ______________________________________                                        1 Character Jump                                                              Start Speed (Step Per Second)                                                                  700       1 Step   Accel                                     Sustain Speed (Steps per Second)                                                               700       0 Steps  Sustain                                   Stop Speed (Step Per Second)                                                                   700       1 Step   Decel                                     10 Character Jump                                                             Start Speed (Steps Per Second)                                                                 700       9 Steps  Accel                                     Sustain Speed (Step Per Second)                                                                1500      2 Steps  Sustain                                   Stop Speed (Steps Per Second)                                                                  700       5 Steps  Decel                                     25 Character Jump                                                             Start Speed (Steps Per Second)                                                                 775      24 Steps  Accel                                     Sustain Speed (Steps Per Second)                                                               3400      2 Steps  Sustain                                   Stop Speed (Step Per Second)                                                                   775      24 Steps  Decel                                     ______________________________________                                    

These values are scored in a look-up table and, in operation, thecomputer 24 determines the jumps necessary between successive charactersand retrieves the relevant profile from the look-up table and suppliesthe relevant profile to the stepper motor so that the new mask apertureis aligned and ready when the laser next fires.

In this particular example the stainless steel mask 26 (67 mm diameter)is constructed from 0.08 mm thick material and all unnecessary mass isremoved from the mask. The mask has only one set of characters andvariations in marked character size and orientation are achievedoptically. The character size is adjusted by changing the imaging lensfocal length (L1 or L2) by means of a drive 34. Dove prisms (P1 and P2)are used to alter the orientation (vertical or horizontal) of thecharacters on the wire, by means of a drive 36.

Most of the wires required to be marked in the industry range in sizefrom 0.75 mm to 6.35 mm in diameter. In order to easily read theidentification marks with the unaided eye, small characters are markedvertically on the narrower wires. As the wires increase in diameter, alarger font is marked horizontally which can be read in the more normalleft to right mode (see FIG. 3).

When a dove prism is rotated about its optical axis, the image of anobject viewed through the prism will be seen to rotate at twice theangular rate of the prism. This principle has been adapted to create thehorizontal and vertical orientations of the marked characters. One prismis set at 45° in order to rotate the image of the mask charactersthrough 90°, the other is set in the more normal horizontal position torotate the images through 180°. This is necessary in this examplebecause of the fixed orientation of the mask characters and the numberof reflecting surfaces in the optical system.

Owing to the fixed repetition rate of the laser and the need for auniform space between characters, the wire must be driven at a constantspeed during marking, but the wire speed is increased betweenidentification marks by suitable control of the tractor drive 16.

The configuration of the marking system of the second embodiment isshown in FIG. 5. In this embodiment the mask assembly, prisms and one ofthe lenses have been removed. The second turning mirror M2 has beenreplaced by a high resolution digital display (DMD) 38 as previouslydescribed above. Light modulated by the DMD 38 is reflected via aturning mirror M3 through a fixed focal length lens L onto the wire 12.This gives the advantage of fast response, a wide range of characterfonts and a reduced number of optical components. The DMD 30 is apixellated structure with typically 500,000 individually addressablepixels each of which may be switched between an "ON" condition in whichit reflects the laser radiation via mirror M3 and lens L along theoptical axis shown, and an "OFF" condition in which it reflects thelaser radiation off axis to a suitable absorber (not shown). In this waya high resolution image, of alphanumeric or bar code form may be writtenon a wire.

What is claimed is:
 1. Laser marking apparatus for marking a successionof characters on a workpiece (12) to form a composite image thereoncomprising:a generally constant pulsed source (26) of laser radiationfor producing a series of laser pulses at a rate of at least 15 Hz,defining a pulsed beam of radiation, a rotatable mask (30) for beingilluminated by said radiation beam and including a plurality ofcharacter apertures spaced angularly around the axis of rotationthereof; drive means (32) for rotating said rotatable mask (30), andcontrol means (24) for controlling said drive means (32) intermittentlyto index said rotatable mask (30) after each of said laser pulses toalign a successive character aperture with said laser beam, thereby toproduce a succession of characters on said workpiece (12) to form saidcomposite image.
 2. Laser marking apparatus according to claim 1,wherein said control means (24) is operable in use to control said drivemeans (32) to maintain said mask (30) substantially stationary for theduration of the laser pulse.
 3. Apparatus according to claim 1, whereinsaid laser source (26) is a solid state laser.
 4. Apparatus according toclaim 3, wherein said laser (26) is a Nd:YAG solid state laser. 5.Apparatus according to claim 1, wherein said laser source (26) includesone or more frequency multiplying means.
 6. Apparatus according to claim1, wherein the laser source (26) provides an output beam in the U.V.waveband 200 nm to 400 nm.
 7. Apparatus according to claim 1, whereinsaid laser beam has a marking fluence of between about 50 mJcm⁻² andabout 2000 mJcm⁻².
 8. Apparatus according to claim 1, wherein the drivemeans (32) comprises a stepper motor and said control means (24)provides a selected drive input profile to said stepper motor to movethe mask (30) between successive angular positions.
 9. Apparatusaccording to claim 8, including means (24) for storing, for each of aplurality of increments of angular movement of said mask (30) to alignsuccessive character apertures in the laser beam, a respective driveprofile.
 10. Apparatus according to claim 9, wherein said profilecomprises a substantially linear acceleration phase, an optionalsubstantially steady speed sustain phase, and a substantially lineardeceleration phase.
 11. Apparatus according to claim 1, wherein the mask(30) comprises a plurality of character apertures making up a singlecharacter set.
 12. Apparatus according to claim 1, wherein the mask (30)is made of thin stainless steel.
 13. Apparatus according to claim 1,including optical transformation means (L₁, L₂) in the beam path beyondthe mask (30), operable to adjust at least one of the size andorientation of a character at the marking plane.
 14. Apparatus accordingto claim 13, wherein the optical transformation means (L, L₂) includesat least one lens means for being moved into the beam to adjust the sizeof the image.
 15. Apparatus according to claim 13, which includesmapping means (P P₂), for being moved into the beam to present thecharacter in a vertical, horizontal or other orientation.
 16. Apparatusaccording to claim 15, wherein said mapping means comprises two doveprisms (P P₂) for presenting the characters in the vertical and thehorizontal orientation respectively.
 17. Apparatus according to claim 1,including means (16) for advancing the workpiece to be marked throughthe apparatus, said means for advancing being controllable to increasethe workpiece speed between identification marks.
 18. A laser markingmethod which comprises marking a succession of characters on a workpiece(12) to form a composite image thereon, said method comprisinggenerating a pulsed beam of laser radiation at a generally constantpulse rate of at least 15 Hz to pass to a rotatable mask (30) carrying aset of mask character apertures thereon, indexing the mask (30)discretely after each of said laser pulses to align a successivecharacter aperture with said laser beam, thereby to produce a compositeimage comprising a succession of characters on said workpiece (12).